Small Animal Radiology and Ultrasound: A Diagnostic Atlas and Text

664 Small Animal Radiolo g y and Ultrasono graphy
lumbar sinus venography, computed tomography (CT), and magnetic resonance imaging
(MRI). The preference as to which technique is used often is based on availability of the
equipment, personal experience, and training.
D I S C O G R A P H Y
Nonionic contrast material may be injected directly into the intervertebral disc space
(using a 20- or 22-gauge spinal needle) to demonstrate prolapse of the intervertebral
disc. 15,18 This technique requires fluoroscopic control of the injection, and, although it can
be useful, it is invasive, of limited value, and not often recommended. The procedure has
been used for the evaluation of disc prolapse. Dorsal extension of the contrast material into
the spinal canal or injection of more than 0.3 ml into the disc were considered evidence of
disc prolapse. 17 Discography combined with epidurography was recommended for evaluation
of dogs with cauda equina compression.
U LT R A S O N O G R A P H Y
Ultrasonography is not often useful for evaluation of the vertebral column. The lumbar
disc spaces can be identified during abdominal ultrasonography; however, the value of this
examination is limited. Ultrasonography has been used to examine the spinal cord intraoperatively.
19-21 This examination has some value in determining the degree of spinal cord
injury that is present following disc prolapse, fracture, or dislocation. Intraoperative ultrasonography
may be valuable also for identification and biopsy of intramedullary lesions,
for evaluation of the central canal, and for evaluation of blood flow within the spinal cord.
C O M P U T E D T O M O G R A P H Y A N D M AG N E T I C R E S O N A N C E I M AG I N G
Both CT and MRI can be used for evaluation of the vertebral column. The vertebral and
spinal cord anatomy can be demonstrated with cross-sectional images providing extremely
useful information. Anesthesia or sedation and careful positioning are mandatory. The
time required to obtain the necessary knowledge and the expense and availability of the
equipment are the major limitations at this time.
CT may be used in conjunction with a myelogram to improve delineation of the subarachnoid
space and spinal cord. A lower contrast concentration than that used for routine
myelography should be injected because of the artifact associated with dense materials.
In a comparison of CT and myelography for vertebral and spinal cord tumors, CT was
superior for evaluating bony change, but myelography was better for classifying spinal cord
lesions. 22
NORMAL ANATOMY OF RADIOGRAPHIC SIGNIFICANCE
G E N E R A L V E RT E B R A L A N AT O M Y
All vertebrae have a basically similar structure. The ventral-most portion is the body, which
is a tubular bony structure. The body has a dense cortex and a marrow cavity consisting of
cancellous bone. The basivertebral veins run through the middle of the vertebral body, producing
a linear radiolucency. These veins connect the ventral venous sinuses on the dorsal
surface of the vertebral body to the caudal vena cava. The transverse processes, which are
bony projections that vary in size and orientation depending upon the specific vertebra,
arise on the left and right sides of the vertebral bodies. Dorsal to the body are bony structures
that combine to form the vertebral arch. These structures include the lamina, pedicles,
and dorsal spinous process. The bony pedicles are the lateral walls of the spinal canal.
These are composed of cortical bone and arise from the most lateral part of the vertebral
body. The pedicles have semilunar defects on their cranial and caudal borders that combine
to form the intervertebral foramina through which the spinal nerves exit the spinal canal.
The laminae that form the roof of the spinal canal connect the dorsal edges of the pedicles.
Dorsal to the laminae is the spinous process, a laterally flattened bony extension that is centered
on the midline and varies in size depending on the specific vertebra being described.
At the junction of pedicles and laminae on the cranial and caudal aspects are paired articular
processes, which articulate with those of the adjacent vertebra. Other bony processes
(i.e., accessory and mammillary) and the costal foveae (for articulation with rib heads) are